The Laser Diode intensity noise as a function of laser current and temperature has been studied in this paper. It is shown that the intensity noise in the current-temperature coordinates forms "islands" of noise amid larger areas of relatively low-noise operation. Although these "islands" of noise exhibit some drift over time, it is generally possible to find operational points characterized by long-term stability and low noise. We show also that the laser noise severely deteriorates the signal-to-noise ratio (SNR) at the receiver --photodiode circuit-- causing random fluctuations of SNR, impairing the performance of interferometric applications. The comparison of noise distributions for isolated laser modules and laser modules incorporated into laser Doppler velocimeters allowed us to reveal design faults contributing to unwanted back reflections deteriorating the overall SNR and operationl stability of the system.
Laser diode intensity noise as a function of laser current and temperature is studied. It is shown that the intensity noise in current-temperature coordinates forms "islands" amid larger area of relatively low-noise operation. Although these "islands" of noise are showing some movement in time, generally it is possible to find an operational point characterized by long-term low noise. We show also that laser noise deteriorates signal-to-noise ratio (SNR) in photodiode circuit causing random fluctuations of SNR. The comparison of noise distributions for isolated laser module and the laser incorporated into Doppler velocimeter gauge allowed us to reveal design faults contributing to back reflection and enhancing the noise.
The statistical properties of photodetector current noise in Doppler and grating schemes are analyzed; in particular, the effect of speckle averaging over the receiving aperture is discussed. It is shown that in the typical configurations of both schemes shot noise prevails over other sources of noise. The dependence of the potential accuracy on the signal-to-noise ratio is established in both schemes. Also, it is shown that the relative frequency spread of the Doppler shifted component and the informational component in the grating scheme can be less than 0.1% with a signal-noise ratio exceeding 100.
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